Usually it is the float and or sending unit in the tank. Over time the float sometimes absorbs fuel and doesn't work as well, usually indicating less than what is there. Or the sender gets corroded or sticky and reads higher or lower than it should.

Only fix that I know of is to replace the unit which means dropping the tank etc. Depending on how many miles you have, if you drop the tank, you might want to consider changing the fuel pump too since it is in the tank and requires the same work. Manytimes the float/sender/pump is all part of one unit.

Alternatively I had one car where the gauge itself was faulty and had to be replaced.

Once in a great while it is a bad wire connection.

Good Luck,

Jim Henderson